Series variable speed drive



May 22, 1945- w. R. HARDING ET AL SERIES VARIABLE SPEED DRIVES FiledNov. 11, 1941 2 Sheets-Sheet l Cur/1:02

Cur/60f INVENTORS M/bamKHa/d/ WITNESSES:

k w M w M m BY M ATTORNEY Cur/60f i 945. w. R. HARDING ET AL 2,376,770

SERIES VARIABLE SPEED DRIVES Filed NOV. ll, 1941 2 Sheets-Sheet 2WITNESSES:

ATTORNEY Patented May 22, 1945 seams VARIABLE snap DRIVE William R.Harding, Murrysvllle, Pa., and Scott H. Hanviile, Jr., Huron, Ohio,assignors to Westinghouse Electric & Manufacturing Company, Earl};aPittsburgh, Pa., a corporation oi Pennsylva Application November 11,1941, Serial No. 418,630

4 Claims. (Cl. 172-239) Our invention relates to adjustable speeddrives, and more particularly to a variable voltage generator and motordrive in which the generator is a series generator without a separatelyor shunt ly at the low speeds and coupled with the provision of lowresidual flux in the generator and still lower residual flux in themotor.

Another broad object of our invention is to excited field winding, andthe motor is also of 5 provide for stable high torque operationoiaseries the series type having substantially the same motor operatedfrom a series generator at low frame size, though not necessarily so, asthe genspeeds and to provide for good speed regulation erator, and thushaving a rating substantially oi the motor at low rapidly variable loadsand low equal to that of the generator. speeds. Our present inventionincludes improvements 1o I is a still further object oi our invention toover our invention disclosed and claimed in our provide for good speedregulation at low speeds pending application entitled Series speedconfrom low loads to high loads regardless of the trol units, filed onOctober 19, 1940, Serial No. rate of variation 01' the loads. 361,934new matured into Patent 2,303,457. It is also an object of our inventionto provide Series motors, of course, are in regular use and in avariable voltage drive having a series genwell known to the trade, butseries generators for erator and a series motor a residual flux-in theoperating motors have always been held very unmotor less than theresidual flux of the generator, satisfactory. For instance, as one oftheir disand means to obtain high torque at low speeds advantages, suchgenerators have a rising voltage up to the stalled rotor condition ofthe motor and characteristic with an increase in load current. 2 goodspeed regulation at low speeds and rapidly This is usually veryunsatisfactory. However, by varying low loads. selecting the propervalues of the constants of Another object 01' our invention is toprovide both the generator and the motor, the motor simple and eflectivemeans for decreasing the speed may be kept substantially constant forany effect of armature reaction tending to displace the speed settingselected regardless of the variations current n combination h tProvision o ood in l ad, speed regulation at low speeds and lightrapidly One broad object of our invention is to provide varying loadswhile utilizing a low residual flux for substantially constant speed ofa series motor in a series ene ato and a Still lower residual energizedfrom a series generator for all speeds flux in the motor. selected forthe motor regardless of the varia- The objects hereinbefore expressedare believed tions in load on the motor and independent of to be merelyillustrative, and many other objects the rate of the variations of suchload on the and advantages will become apparent from a motor. study ofthe following specification when con- Another object of our invention isto provide for sidered with the drawings accompanying this stable hightorque operation of a series motor, specification and in which drawings,operated from a series generator, at low motor Figural is a diagrammaticshowing of our inspeed and rapid load variation. vention;

Another object oi our invention as hereinafter Figs. 2, 4. and 5 how apl r i y f rv disclosed is to provide for good speed regulation helpfulin illustrating some of the characteristics with rapidly varying loads,and also stable operaof our series drive; tion at light loads on motor.a Fig. 6 is a diagrammatic showing of the modi- It is also an object ofour invention to provide fication showing a more comprehensivecombinaduring rapid changes of load good speed regulation; and tion in avariable voltage drive utilizing a series Fig. 7 shows our drive in itssimplest diagramgenerator and a series motor, a low residual flux maticform. in the generator and a low residual flux in the In Fig. 1, Gdesignates a series generator and motor but wherein the residual flux ofthe motor M a Series motor- The tWO y a oct ic mais less than theresidual flux of the generator. chines G and M are preferably. t o h notneces- A still further object of our invention is the sarily, of thesame frame size and thus have comprovision of the substantially constantspeed for 5 parable ratings and are otherwise preferably gena seriesmotor energized from a series generator erally alike in structure. forall speeds selected for the motor regardless of As W e S en f o g. thegenerator is the variations in load and the rate of variations driven byu t e stant peed alternating 1 in load on the motor coupled with theprovision current motor I representing an induction motor for increasingthetorque of the motor, particularconnected through a suitable lineswitch I, controlled by means of push-button control to a suitablesource of alternating current. The generator G, of course, may be'drivenat some selected constant speed in any suitable manner, and it is notone of the features of our invention that a constant speed inductionmotor need be used.

In the showing in Figs. 1 and 6, we have shown the motor and thegenerator of the four-pole type, but it is readily apparent that ourinvention is not limited to a four-pole construction but may beapplicable to machines of the two-pole type or of the type having morethan four poles.

To improve the speed regulation, particularly at light loads, andrapidly varying loads, and also at low speeds and light loads andrapidly varying loads and heavy loads which may also vary rapidly, ourtheoretical considerations showed us that this could be accomplished byincluding our present contribution to the art.

Theoretical considerations also showed that better speed regulation athigh loads and low speeds could be obtained if the current sheet wereprevented from shifting because of armature reaction, and still betterregulation could be obtained if a control were provided wherein thevoltage characteristic of the generator is not susceptible to rapidvariations in load that is a control which would tend to maintain thegenerator voltage when the current demand of the motor was increasingand in which the speed characteristic of the motor is not susceptible torapid variations in load, or both. By the term current sheet is meantthe effective region on the surface of the brushes of a machine at whichit may be considered that the entire current flow is concentrated.

In Fig. 1, we show interpoles or commutating poles on the generatorhaving a high magnetic capacity so that the pole pieces for thelnterpoles do not become readily saturated. Since the generator for thepast operation is selected to have a low residual, the burden ofmaintaining its voltage at low motor speeds and heavy loads falls moreand more on the generator series field windings. Since the voltage isnecessarily low at the low speeds, the armature reaction distorts orshifts the current sheet so much that the generator cannot maintain itsvoltage. The result is poor regulation at low motor torques, at highloads and low speeds. We provide in our present combination theadditional improvement of using interpoles for the generator having ahigh magnetic capacity. At low speeds and heavy loads, the interpoles donot saturate and the current sheet thus remains fixed. In our presentcase, we provide still further improvements to the operatingcharacteristics of the system by preventing rapid variations in load todisadvantageously affect the operation of the system. In Figs. 1 and 7,we show a reactor 2 having a closed magnetic circuit and having one ofits coils, as coil 3, connected in series with a generator field 4 andhaving its other coil, as 5, connected in series with the adjustablerheostat G for controlling the voltage of generator G. This reactor isin eifect a transformer and is selected to have a ratio so that the mostcorrection is provided at the point most needed. For example, at lowmotor speeds and high torques it is desirable that the generator voltagebe as high as possible to avoid a. dropping off in motor speed. If forthe particular operating conditions it is desired to have the speedmaintained at this point, a transformer of a suitable ratio to providethe most correction for this operating condition would be selected. Whenthere is a rapid variation in load on the motor, the current flowing inthe generator and the motor armature windings, the series field windingsof the two machines, the rheostat and the reactor coils, changesrapidly. This loop circuit is essentially a very simple Ward-Leonardsystem, and in which system the rheostat 8 is used to effect the speedcontrol for the motor. In this novel type or series Ward- Leonardsystem. if no reactor is used, the current in the loop circuit will,during the change, take the path of least resistance, and since theseries field winding 4 is an inductive circuit, the load current willflow through the rheostat 6 rather than through the field 4. Forinstance, if the load on the motor is rapidly increased, the excitationof the motor is increased, therefore tending to decrease its speed, andin the absence of the reactor 2, the current flowing through therheostat rather than the field l, the voltage of the generator remainssubstantially constant, with the result that there is a rapid decreasein the motor speed. True, this may only be temporary but for someapplications, it is a very unsatisfactory operation. For instance, ifthe load increases, the generator for the moment will be operating at lof its saturation curve, see Fig. 2, and after the increase of the loadwill be operating at B.- At the same time, the load current on the motorvaries from 9, Fig. 4, to 10 which means that the speed of the motor,referring to Fig. 3, will drop, 11 to 12. If these changes are allreferred to time, as is indicated by the curves in Fig. 5, the loadcurrent variations will be indicated by the curve It, and during a rapidchange in load current will move, substantially as indicated, from 14 to15 in a given time. This shows that a rather abrupt increase in currentoccurs which reaches a peak value at the time limit of the centermostdotted line and then drops of! slightly. The generator voltagerepresented by curve It will at the instant the load begins to increasestill for sometime, measured by the difierence in position of points I!and I8, maintain its voltage. The speed variation of the motor, asrepresented by curve l9, will vary as indicated. With the reactor in theloop circuit any change, as for instance, an increase in the load on themotor, will require an immediate increase in the current through thefield winding 4. The generator thus immediately attains a new desiredhigher voltage, and whatever efiect may have been produced on the fieldof the winding of the motor is counteracted by the increase in voltageof the generator.

Our reactor is, of course, of particular utility when the motor isoperating at low speed where any abrupt variations in load might wellreduce the generator voltage to a value so low as to stop the motor.

Our. reactor thus provides an improved operation for our entire systemutilizing low residual machines and utilizing machines wherein thegenerator has a higher low residual than the low residual of the motor.By low residual is meant a residual that is low with reference to theno-load saturation flux of the machine, namely, lower than is normal fordynamoelectric machines-of conventional characteristics.

Our special arrangement of the reactor has, of course, utility for themotor field as well as for the generator field. In Fig. 6, we show asystem arranged where the same reactor may be used both for thegenerator and for the motor. For instance, with the switch 20 disposedin the posithe loop circuit, including on the basis of constant voltage,from 2,376,770 tion shown, the reactor is used in conjunctionwith thegenerator field 4 and may be retained in that position until the verymaximum speed of the motor has been obtained by means of the control ofthe field of the generator by rheostat 6. However, when all of theresistor sections of rheostat 6 have been inserted, and still higherspeeds are to be obtained from the motor M, switch 20 is thrown to itsother position, thereby leaving the generator field winding for maximumexcitation and for the moment the motor field for maximum excitation.The rheostat 6 may then be adjusted to get still higher speeds from themotor, but regardless of the position of switch 6, the advantageousfunction of the reactor 2 is always obtained. For the first stage, it isobtained by providing voltage changes with load,

and for the second stage, the reactor prevents rapid field currentchanges and thus changes in speed of the motor with rapid changes inload. This will be better understood when it is seen that in its secondposition the switch 20 connects the coil 8 of the reactor between thegenerator series field l and the motor series field M. The other reactorcoil 5 with the rheostat 6 in series therewith is connected in parallelwith the reactor coil 4 and the motor series field M. Inductivecircuits.

are thus provided in both paralleled branches and due to thecharacteristics of inductive circuits any changes in motor fieldcurrents are opposed. .With the arrangement we have thus shown in thisapplication, good speed regulation is obtained over a wide range ofloads for any selected speed regardless of the rate of change of theload.

The reactor makes possible speed control in a series drive by decreasingthe motor series field excitation A method of speed control notheretoiore possible is thus obtained. The speed range of the equipmenthas thus been widened without any increase in the physical size of themachines particularly i'or those loads demanding constant horsepoweroperation.

The reactor also improves, that is, stabilizes the operation for steadylight load conditions.

We are aware that others, particularly after having had the benefit ofour teachings, may devise still further circuits and other similararrangements to .obtain the novel results. We, therefore, do not wish tobe limited to the specific showing made but wish to be limited only r vby the scope oi the claims hereto appended.

We claim as our invention:

1. In a series drive, in combination, a series generator having anarmature winding, a series field'winding, a series motor, of the samegeneral design and capacity as the generator, connected in a loopcircuit to the series generator to be electrically driven thereby; anadjustable resistor shunting the generator series field winding forcontrolling the excitation of the generator series field winding; areactor having two coils one connected in series circuit relation withthe series field winding and the other in series circuit with theadjustable resistor, said two series circuits being connected inparallel; and means for driving the series generator at substantiallyconstant speed.

2. In a series drive, in combination, a series enerator having anarmature winding, a series field winding, 9. commutating field windinghaving a magnetic-circuit of a magnetic capacity sufficiently high so asnot to become saturated by high currents traversing the commutatingfield winding thereby preventing a, shifting-of the current sheet byarmature reaction, a series motor, of the same general design andcapacity as the generator, connected in a loop circuit to the seriesgenerator to be electrically driven thereby; an adjustable resistor forcontrolling the excitation of the generator series field winding; areactor having two coils one connected in series circuit relation withthe series field winding and the other in series circuit with theadjustable resistor, said two series circuits being connected inparallel; and means for driving the series generator at substantiallyconstant speed.

' 3. In a series drive, in combination, a series generator having anarmature winding and a series field winding; a series motor having anarmature winding and a series field winding connected in a loop circuitto said generator to be electrically driven thereby; an adjustableresistor for controlling the excitation of the generator series fieldwinding; a reactor having two coils, one coil of said reactor beingconnected in series with the generator series field winding and theother coil being connected in series with the adjustable resistor, saidseries circuit including the one coil of the reactor and the adjustableresistor being connected in parallel to the series circuit including thegenerator series field and the other coil of the reactor; means forselectively interconnecting said reactor with either the generatorseries field winding or the motor field winding in the recited relation;and means for driving the I generator at substantially constant speed.-

4. In a series drive, in combination, a series generator having anarmature winding and a series field winding; 9. series motor having anarmature winding and a series field winding connected in a. loop circuitto said generator to be electrically driven thereby;. said generatorhaving a commutating field winding having a magnetic circuit of amagnetic capacity sufiiciently high so as not to become saturated byhigh currents traversing the commutating field winding therebypreventing a shifting oi. the current sheet by armature reaction;anadjustable resistor for controlling the excitation of the generatorseries field winding; a reactor having two coils one coil of saidreactor being connected in series with the generator series fieldwinding and the other coil being connected in series with the adjustableresistor, said series circuit including the one coil of the reactor andthe adjustable resistor being connected in parallel to the seriescircuit including the generator series field and the other coil of thereactor; means for selectively interconnecting said reactor with eitherthe generator series field winding or the motor field winding in therecited relation; and means for driving the generator at substantiallyconstant speed.

R. HARDING. SCOTT H. HANVIILE, JR.

